Accurate timekeeping has proven to be useful for many modern technologies. For example, the Global Positioning System (GPS), various computing devices and telecommunications networks all rely on accurate clocks. Compared to prior clocking devices, atomic clocks provide highly accurate timekeeping. The first atomic clock was an ammonia maser device built in 1949 at the U.S. National Bureau of Standards, now known as the National Institute of Standards and Technology (NIST). As a reflection of the importance of accurate timekeeping to modern commerce, NIST is an agency within the U.S. Department of Commerce.
Atomic clocks rely photons emitted by an electron transition from a higher energy level to a lower energy level as constrained by the laws of physics in general, and the laws of quantum mechanics in specific. Atomic clocks have been built based on the particularly precise “hyperfine” transitions often associated with the elements cesium, rubidium and hydrogen. For example, NIST uses the NIST-F1 atomic clock as one of the national primary time and frequency standards of the United States. The NIST-F1 is a type of atomic clock known as a cesium fountain clock, located at NIST facilities in Boulder, Colo., and serves as the United States' primary time and frequency standard. While the NIST-F1 is highly accurate, it is not a perfect timekeeping device as all known timekeeping devices are imperfect.